Shared memory systems cover a broad spectrum, from systems that maintain consistency entirely in hardware to those that do it entirely in software. We have studied the hardware end of the spectrum in some detail and have given a brief summary of the software end (page-based distributed shared memory and object-based distributed shared memory). In Fig. 6-10 the spectrum is shown explicitly.

Fig. 6-10. The spectrum of shared memory machines.

On the left-hand side of Fig. 6-10 we have the single-bus multiprocessors that have hardware caches and keep them consistent by snooping on the bus. These are the simplest shared-memory machines and operate entirely in hardware. Various machines made by Sequent and other vendors and the experimental DEC Firefly workstation (five VAXes on a common bus) fall into this category. This design works fine for a small or medium number of CPUs, but degrades rapidly when the bus saturates.

Next come the switched multiprocessors, such as the Stanford Dash machine and the M.I.T. Alewife machine. These also have hardware caching but use directories and other data structures to keep track of which CPUs or clusters have which cache blocks. Complex algorithms are used to maintain consistency, but since they are stored primarily in MMU microcode (with exceptions potentially handled in software), they count as mostly "hardware" implementations.

Next come the NUMA machines. These are hybrids between hardware and software control. As in a multiprocessor, each NUMA CPU can access each word of the common virtual address space just by reading or writing it. Unlike in a multiprocessor, however, caching (i.e., page placement and migration) is controlled by software (the operating system), not by hardware (the MMUs). Cm* (Jones et al., 1977) and the BBN Butterfly are examples of NUMA machines.

Continuing along the spectrum, we come to the machines running a page-based distributed shared memory system such as IVY (Li, 1986) and Mirage (Fleisch and Popek, 1989). Each of the CPUs in such a system has its own private memory and, unlike the NUMA machines and UMA multiprocessors, cannot reference remote memory directly. When a CPU addresses a word in the address space that is backed by a page currently located on a different machine, a trap to the operating system occurs and the required page must be fetched by software. The operating system acquires the necessary page by sending a message to the machine where the page is currently residing and asking for it. Thus both placement and access are done in software here.

Then we come to machines that share only a selected portion of their address spaces, namely shared variables and other data structures. The Munin (Bennett et al., 1990) and Midway (Bershad et al., 1990) systems work this way. User-supplied information is required to determine which variables are shared and which are not. In these systems, the focus changes from trying to pretend that there is a single common memory to how to maintain a set of replicated distributed data structures consistent in the face of updates, potentially from all the machines using the shared data. In some cases the paging hardware detects writes, which may help maintain consistency efficiently. In other cases, the paging hardware is not used for consistency management.

Finally, we have systems running object-based distributed shared memory. Unlike all the others, programs here cannot just access the shared data. They have to go through protected methods, which means that the runtime system can always get control on every access to help maintain consistency. Everything is done in software here, with no hardware support at all. Orca (Bal, 1991) is an example of this design, and Linda (Carriero and Gelernter, 1989) is similar to it in some important ways.

The differences between these six types of systems are summarized in Fig. 6-11, which shows them from tightly coupled hardware on the left to loosely coupled software on the right. The first four types offer a memory model consisting of a standard, paged, linear virtual address space. The first two are regular multiprocessors and the next two do their best to simulate them. Since the first four types act like multiprocessors, the only operations possible are reading and writing memory words. In the fifth column, the shared variables are special, but they are still accessed only by normal reads and writes. The object-based systems, with their encapsulated data and methods, can offer more general operations and represent a higher level of abstraction than raw memory.

Item

Single bus

Switched

NUMA

Page based

Shared variable

Object based

Linear, shared virtual address space?

Yes

Yes

Yes

Yes

No

No

Possible operations

R/W

R/W

R/W

R/W

R/W

General

Encapsulation and methods?

No

No

No

No

No

Yes

Is remote access possible in hardware?

Yes

Yes

Yes

No

No

No

Is unattached memory possible?

Yes

Yes

Yes

No

No

No

Who converts remote memory accesses to messages?

MMU

MMU

MMU

OS

Runtime system

Runtime system

Transfer medium

Bus

Bus

Bus

Network

Network

Network

Data migration done by

Hardware

Hardware

Software

Software

Software

Software

Transfer unit

Block

Block

Page

Page

Shared variable

Object

Fig. 6-11. Comparison of six kinds of shared memory systems.

The real difference between the multiprocessors and the DSM systems is whether or not remote data can be accessed just by referring to their addresses. On all the multiprocessors the answer is yes. On the DSM systems it is no: software intervention is always needed. Similarly, unattached global memory, that is, memory not associated with any particular CPU, is possible on the multiprocessors but not on the DSM systems (because the latter are collections of separate computers connected by a network).

In the multiprocessors, when a remote access is detected, a message is sent to the remote memory by the cache controller or MMU. In the DSM systems it is sent by the operating system or runtime system. The medium used is also different, being a high-speed bus (or collection of buses) for the multiprocessors and a conventional LAN (usually) for the DSM systems (although sometimes the difference between a "bus" and a "network" is arguable, having mainly to do with the number of wires).

The next point relates to who does data migration when it is needed. Here the NUMA machines are like the DSM systems: in both cases it is the software, not the hardware, which is responsible for moving data around from machine to machine. Finally, the unit of data transfer differs for the six systems, being a cache block for the UMA multiprocessors, a page for the NUMA machines and page-based DSM systems, and a variable or object for the last two.